As laptop computers, cellular telephones, and other such electronic devices are handled, they can become soiled with oil from the hands of the user or with cigarette tar, and dust and so forth can adhere via these substances. Also, when oil from the hands adheres to an electronic device, it tends to foster the proliferation of germs and the like on the surface of the device. Furthermore, if sebum, cigarette tar, germs, and the like are not removed, they can adversely affect the appearance of electronic devices, which in turns makes the devices look dirty. With laptop computers, cellular telephones, and other electronic devices, the need to avoid this is particularly great for the housing and the transparent cover used to protect the display screen. In addition, the growing concern over microbes in our living environments has led to the requirement that the housings, control keys, and so forth of laptop computers, cellular telephones, and other electronic devices be antimicrobial. Because of this, there has been a need in the field of electronic devices for some antimicrobial/anti-soiling technology for dealing with the problems caused by sebum, cigarette tar, germs, and so forth.
The photocatalytic function of certain semiconductor substances such as titanium oxide (TiO2) has come under scrutiny in recent years, and it is known that an antimicrobial action, anti-soiling action, and so on can result from this photocatalytic function. Photocatalytic semiconductor substances generally absorb light having energy corresponding to the band gap between the valence band and the conductor band, causing electrons from the valence band to make a transition to the conduction band, and this electron transition produces holes in the valence band. Electrons in the valence band have the property of moving to substances adsorbed to the surface of the photocatalytic semiconductor, and this can result in the chemical reduction of the adsorbed substances. Holes in the valence band have the property of stripping electrons from a substance adhering to the surface of the photocatalytic semiconductor, and this can result in the oxidation of the adsorbed substances.
With photocatalytic titanium oxide (TiO2), electrons that have made the transition to the conduction band reduce the oxygen in the air, producing a superoxide anion (.O2−) Along with this, the holes produced in the valence band oxidize adsorbed water on the surface of the titanium oxide, producing hydroxy radicals (.OH). Hydroxy radicals are extremely oxidative. Accordingly, when an organic compound, for instance, is adsorbed to photocatalytic titanium oxide, it may be decomposed into water and carbon dioxide by the action of the hydroxy radicals. Because it is capable of promoting such oxidative decomposition reactions in organic substances on the basis of its photocatalytic function, titanium oxide is widely used in antibacterial agents, disinfectants, antifouling agents, deodorants, environmental cleaning agents, and so forth.
Titanium oxide (TiO2) is colorless. Therefore, this colorless titanium oxide is sometimes formed into a thin layer by sputtering on the surface of a certain object for the purpose of imparting antimicrobial activity, for example, without affecting the member aesthetically. When formed in a thin layer by sputtering, a titanium oxide film is itself substantially transparent.
However, when a titanium oxide thin film is formed on a glass surface, an interference fringe is often produced when the light rays transmitted through the titanium oxide thin film and the glass interfere with each other, resulting in a loss of transparency in the glass. This is because the refractive index of titanium oxide is about three times that of glass. It is possible to reduce the occurrence of this interference fringe by making the titanium oxide film thinner, but the thinner the film, the less antimicrobial activity had by the titanium oxide film. If the titanium oxide film becomes too thin, sufficient antimicrobial activity may not be obtained on a glass surface.
Also, titanium oxide itself does not adsorb substances well to its surface. Therefore, to obtain sufficient photocatalytic function (oxidative decomposition action), and in turn, antimicrobial action, anti-soiling action, and so forth, in titanium oxide on the basis of its photocatalytic function, it is necessary to increase the contact efficiency between the titanium oxide and the material that is to be oxidatively decomposed.
A technique for increasing the contact efficiency between titanium oxide and a material that is to be decomposed has been disclosed in JP-A No. 2000-327315, for example. This publication discloses a photocatalytic apatite produced by compounding on the atomic level of, for example, titanium oxide (which has a photocatalytic function) and calcium hydroxyapatite (CaHAP) (which is particularly good at adsorbing proteins and other organic substances). In specific terms, this photocatalytic apatite is titanium-modified calcium hydroxyapatite (Ti—CaHAP) having a crystal structure in which part of the calcium that makes up the CaHAP (Ca10(PO4)6(OH)2) has been substituted with titanium. A titanium oxide-like partial structure resembling the chemical structure of photocatalytic titanium dioxide is formed at the site where the titanium is introduced. Because a titanium oxide-like partial structure capable of exhibiting a photocatalytic function is present in the crystal structure of the CaHAP, which adsorbs organic substances so well, the contact efficiency between the organic substance (the material to be decomposed) and the titanium oxide-like partial structure is effectively increased. Therefore, this titanium oxide-like partial structure is able to efficiently oxidize and decompose organic substances such as oil from the hands or bacterial cell membranes through its photocatalytic function.
According to JP-A No. 2000-327315, the photocatalytic apatite is obtained in the form of a powder. A thin film of photocatalytic apatite can be formed on a specific substrate by sputtering, using a sputtering target made from this photocatalytic apatite powder. Techniques for forming a film of apatite material by sputtering are disclosed, for example, in JP-A No. 10-72666 and JP-A No. 10-328292. Ti—CaHAP, which is an example of photocatalytic apatite, is colorless and has a refractive index comparable to that of glass. Accordingly, when a Ti—CaHAP is formed by sputtering on a glass surface, substantially no interference fringe is produced because there is almost no interference in the light transmitted through the Ti—CaHAP film and the glass. Thus, with photocatalytic apatite applied to a glass surface by sputtering, it is sometimes possible to preserve good transparency in the glass without having to reduce the film thickness excessively.
Nevertheless, it is known that with prior art, the photocatalytic activity of a sputtered photocatalytic apatite film is decreased considerably compared to that of the photocatalytic apatite prior to film formation. The first step in sputtering is to accelerate the inert gas ions serving as the sputtering gas, so that these ions collide with a target made from the substance to be made into a thin film. This causes the substance to be scattered from the target surface. The scattered substance is deposited on a substrate disposed across from the target, and as a result a thin film is formed on the substrate. When a conventional photocatalytic apatite film formation technique employing sputtering is used to form a film, it is surmised that in the course of the scattering of the photocatalytic apatite from the target, the crystal structure of the apatite is destroyed to the extent that there is an excessive decrease in the photocatalytic function thereof. Therefore, up to now it has been impractical to employ sputtering as the film formation method in the formation of a photocatalytic apatite film exhibiting a photocatalytic function.